This invention generally relates to the manufacture of substrates having translucent or shadowed watermarks printed thereon. In particular, the invention relates to the manufacture of paper having chemical watermarks printed thereon.
Particularly in office operations of commercial enterprises, it is often desirable to have writing stationary and other forms of business and professional papers watermarked. Watermarks are defined as translucent or opacified areas in a sheet of paper that are formed into identifiable designs such as company names, logos and seals, and are used in paper for security and prestige.
In accordance with one conventional type of shadow marked paper, shadow watermarks are formed by decreasing the density of the paper fibers in a portion of a sheet of paper relative to the density of remaining portions of the paper. Such shadow watermarks are conventionally formed in the papermaking process by contacting a wet web of paper on a Four-drinier paper machine with a dandy roll (i.e., a metal mesh roll) having an indented or recessed surface conforming to the watermark design image to be formed on the paper. During such contact the paper fibers accumulate in the indented or recessed surface, resulting in decreased density of the paper fibers in that localized area. Such shadow watermarks tend to be relatively more opaque than the remainder of the paper, i.e., they tend to transmit less light relative to the remainder of the paper.
Similarly, translucent watermarks can be formed using the reverse procedure in which a dandy roll with a raised design contacts the wet paper to compress the paper fibers and increase their density in that localized area. The compressed area on the substrate becomes relatively more translucent (due to reduced entrapped air and light refraction) and makes the watermark design visible when light is passed through the substrate.
In accordance with a more recent method for manufacturing watermarked paper by impregnating the paper with a solution containing a chemical agent that changes the light-transmitting properties of the impregnated areas. In the case of translucent watermarks, the areas to be watermarked are impregnated with a chemical composition having the ability to render opaque or semi-opaque paper more translucent or substantially transparent, i.e., a solution containing translucentizing agent. In addition, the chemical composition should not alter the surface of the paper adversely. For example, it must not render the surface glossy in the impregnated area and must not alter the erasability characteristic of the paper. Also, the chemical composition must withstand aging without discoloration and must not become indistinct through migration of the chemicals or otherwise. Finally, aside from low cost, the chemically watermarked area must accept typing, penciling, printing and writing inks without adverse effects such as feathering or skipping.
In the case of chemical shadow watermarks, the paper is impregnated with an opacifying agent instead of a translucentizing agent. The chemical shadow watermarks must satisfy the conditions set forth in the previous paragraph.
The conventional chemical watermarking process uses the flexo printing process to induce a translucentizing (or opacifying) chemical polymer into the substrate. The flexo printing plate contains the watermark design and imparts the design to the printed area to form the watermark.
The cost to produce watermarked papers on a paper machine is high, particularly in small (e.g., 500 lb.) quantities. The current method of producing chemical watermarks reduces the cost of manufacture, but still is not profitable in small quantities. Thus there is a need for a method of applying chemical watermarks which will further reduce the cost of manufacturing in small quantities.
The present invention is a method and an apparatus for applying chemical watermarks to a substrate, e.g., paper, using a digital printer which applies ink in a dot matrix pattern. In accordance with the preferred embodiment of the invention, the digital printer is an ink jet printer. Any type of ink jet printer can be used, including ink jet printers of the thermal (bursting vapor bubbles), piezoelectric and continuous (ultrasound) varieties.
The invention allows watermarks to be printed digitally and on demand. The application of chemical watermarks using a digital printer allows a great degree of customization and the production of low volumes at an affordable cost to the user. In addition, the digital aspect allows production of watermarked papers in a shorter period of time as compare to conventional manufacturing practices involving the application of chemicals.